# Patent

## Title
Method for quantifying the risk of falling of an elderly adult using an instrumented version of the FTSS test

## Abstract

Methods and systems may provide for estimating falls risk based on inertial sensor data collected during a Five Times Sit-to-Stand (FTSS) test. In an embodiment, a classifier model may be trained with inertial sensor data collected from a sample of people performing the FTSS test and their self-reported falls history. In an embodiment, one or more features related to steadiness or smoothness of the person's movement may be calculated. In an embodiment, one or more features related to timing of the FTSS test, such as a total time taken to complete the FTSS test or to complete individual sit-stand-sit (SSS) phases of the test, may be calculated. In an embodiment, supervised pattern recognition techniques may train the classifier model to classify a person as being likely to fall or not being likely to fall based on FTSS-related feature values collected from that person.

## Background
BACKGROUND
Technical Field
Embodiments generally relate to quantifying a person's risk of falling.
Discussion
Falls are a very common problem in the older population. About 30% of community dwelling elderly people 65 years or older fall each year. About 12% fall at least twice. Such falls may lead to injury, disability, or even death. Estimating a person's risk of falling may allow early clinical intervention to treat those having a high risk of falling.
SUMMARY
An embodiment of this invention relates to analyzing movement-related data that are collected from a person performing the Five Times Sit-to-Stand (FTSS) test. The FTSS test assesses a person's mobility and balance. During the FTSS, a person must fully stand up and sit back down five times as quickly as possible. The total time taken to perform the FTSS test may provide some indication of falls risk. This embodiment is an instrumented FTSS (iFTSS) technique that further or alternatively relies on the movement-related data collected from the FTSS test. For this iFTSS technique, one or more (e.g., two) inertial sensors, such as accelerometers, may be attached to the person performing the test. Acceleration data associated with the FTSS test may be received from the accelerometers. Other data, such as a total time taken to complete the FTSS test, or the person's age, may also be received.
In an embodiment, the acceleration data or other FTSS-related data may be used as part of a supervised pattern recognition technique that generates an iFTSS model (e.g., a function) for classifying a person as being likely or not likely to fall based on his or her FTSS-derived data. For example, the model may be generated from a sample group of people who perform the FTSS test. Acceleration data collected from each person in the sample may be associated with that person's self-reported falls history (e.g., of the last five years), which may include whether that person has fallen or how many times that person has fallen. A fall may be defined as an unexpected loss of balance resulting in coming to rest on the floor, the ground, or an object below the knee level. A classifier model, such as a linear discriminant model, may be trained with the acceleration data collected from each person in the sample and the person's corresponding falls history. In an embodiment, the trained classifier model may then be used to classify a person outside the sample as being likely to fall or not likely to fall based on FTSS-derived data collected from that person. The trained classifier model may allow a person to estimate his or her risk of falling in a non-clinical setting.
These and other embodiments are described in more detail below.



BRIEF DESCRIPTION OF THE DRAWINGS
The various features of the embodiments will become apparent to one skilled in the art by reading the following specification and appended claims, and by referencing the following drawings, in which:
FIG. 1 illustrates an example setup for an individual to perform the Five Times Sit-to-Stand (FTSS) test and for collecting inertial sensor data from the individual.
FIG. 2 illustrates example operations for estimating falls risk from inertial sensor data collected from a FTSS test.
FIG. 3 illustrates example acceleration data collected from a FTSS test.
FIG. 4 illustrates example performance metric values of various falls risk estimation models.
FIG. 5 illustrates example results generated by various falls risk estimation models.



DETAILED DESCRIPTION
Embodiments may provide for a system and method for generating classifier models (e.g., classifier functions) or other models that estimate a person's falls risk based on FTSS-derived data collected from that person. The FTSS-derived data may include inertial sensor data, and may also include clinical parameters such as total time taken to complete the FTSS test or such as age of the person performing the FTSS test.
In an embodiment, a person performing the FTSS test may be asked to refrain from vigorous exercise on the previous day and on the day of the test. In an embodiment, the person may be asked to eat a light breakfast, light lunch, and a light snack, and to refrain from drinking caffeinated drinks. In an embodiment, one or more times at which the person took medication may be recorded. During the test, the person may be asked to keep his or her arms folded and to fully stand up and sit back down five times as quickly as possible.
In an embodiment, inertial sensor data associated with the FTSS test may be received from one or more inertial sensors attached to the person performing the test. For example, as illustrated in FIG. 1, a first tri-axial accelerometer 105 may be attached at the person's anterior thigh, and a second tri-axial accelerometer 103 may be attached at the person's sternum. The first tri-axial accelerometer 105 may be positioned such that one of its axes aligns with a longitudinal line of the person's femur (i.e., with the person's femoral axis). The second tri-axial accelerometer 103 may be positioned such that its axes measure the person's acceleration along his or her superior-inferior (SI) axis, anterior-posterior (AP) axis, and medial-lateral (ML) axis.
In an embodiment, each accelerometer 103, 105 may be programmed to sample each axis at a rate of 102.4 Hz using custom developed TinyOS firmware.
FIG. 2 illustrates a method 200 for generating and training a classifier model for estimating falls risk based on FTSS-derived data.
At operation 201, data from one or more inertial sensors attached to a person performing a FTSS test may be received. For example, acceleration data may be received from accelerometers 103 and 105.
In an embodiment, data from the one or more inertial sensors may be collected through a single session, or may be collected through multiple sessions over a period of several minutes, hours, or days. For example, the inertial sensor data may be collected from a person who is asked to perform multiple FTSS tests throughout a single day.
At operation 202, self-reported falls history information of the person may be received. The falls history may indicate whether the person has experienced a fall, which may include coming to rest on a floor or other lower level, regardless of whether an injury was sustained. The falls history may exclude events that were a result of a major intrinsic event or overwhelming hazard. Based on a person's self-reported falls history, the person may be classified as a faller if he or she had experienced multiple falls or one fall requiring medical attention in the 12 months preceding the report. In an embodiment, a person may be classified as a faller based on falls before the previous 12 months if the falls were accompanied by more serious symptoms, such as a bone fracture or a blackout.
In an embodiment, values of clinical parameters, such as a person's age, BMI, and grip strength may be collected. In an embodiment, the person's gender may be recorded. An example of clinical parameter values among fallers and non-fallers is illustrated in Table 1. In the example, univariate analysis of variance (ANOVA) was used to compare the ages of fallers and non-fallers. The analysis showed no significant difference in age between fallers and non-fallers. One or more of the clinical parameters may be used as a feature in the classifier model.











TABLE 1









Univariate





analysis of



Non-fallers (M/F)
Fallers (M/F)
variance



















N
9/11
7/12
p > 0.05


Age (yrs)
70.43 ± 6.88/66.32 ± 4.42
74.41 ± 5.22/76.44 ± 8.28
p < 0.05


BMI (m/kg2)
28.62 ± 2.65/27.01 ± 2.76
25.35 ± 2.24/26.27 ± 4.54
p > 0.05


Grip (N)
73.46 ± 30.88/57.83 ± 10.35
83.05 ± 12.19/35.03 ± 12.22
p > 0.05









At operation 203, the received data may be calibrated and filtered to derive acceleration vectors with respect to unit axes of the one or more inertial sensors. For example, raw inertial sensor data from accelerometers 103 and 105 may be bandpass filtered with a frequency range of 0.1-5 Hz with a zero-phase 8th order Butterworth filter with a 50.2 Hz corner frequency. In an embodiment, the one or more inertial sensors may include a gyroscope.
At operation 205, temporal events may be detected based on the inertial sensor data. For example, the acceleration data received from accelerometer 105, which is attached to a person's thigh, may be used to identify times at which the person performing the FTSS test was standing or was sitting, or to identify transitions between standing and sitting (or between sitting and standing). In an embodiment, such events may be determined based on identifying cycles in the acceleration data. As illustrated in FIG. 3, each cycle may include a point of minimum acceleration, or mid-stand point 305. A value of the mid-stand point 305 may be used to base a transition threshold between sitting and standing (or vice versa). For example, a threshold for indicating such transitions may be based on a percentage of the value of the mid-stand point 305. In a more specific example, transitions between sitting and standing may be based on thresholds of 20% and 80% of the mid-stand point value, or 0.2 AMS and 0.8 AMS, respectively. In the example, a decrease in acceleration from zero (and an increase in absolute value) corresponds to a transition between sitting and standing. As illustrated in FIG. 3, decrease of the acceleration below 0.2 AMS indicates a start 301 of the transition between sitting and standing, while decrease of the acceleration past 0.8 AMS indicates an end 303 of the transition between sitting and standing. Past the mid-stand point, increase of the acceleration toward zero corresponds to a transition between standing and sitting. Increase of the acceleration past 0.8 AMS indicates a start 307 of a transition between standing and sitting, and increase of the acceleration above 0.2 AMS indicates an end 309 of the stand-sit transition. Data indicative of the person's femoral acceleration may thus identify cycles of sit-stand-sit (SSS) phases. Identification of the SSS phases, sit-stand transitions, and/or stand-sit transitions may allow other inertial sensor data to be corresponded to events in the FTSS test.
At operation 207, a value of one or more features or parameters related to timing of the FTSS may be calculated. The one or more parameters may include a total time taken to complete the FTSS, a time taken to complete individual sit-stand-sit (SSS) phases, or any other time-related feature. In an embodiment, the one or more features may include a time taken for a sit-stand transition (e.g., from start to finish) or for a stand-sit transition. In an embodiment, the one or more time-related features may be calculated based on sitting and standing events detected from the inertial sensor data. For example, the total time may be calculated as a difference between a time corresponding to the start of the first sit-stand transition and a time corresponding to the end of the fifth stand-sit transition, as detected from the femoral acceleration data.
At operation 209, a value of one or more features or parameters related to acceleration may be calculated. In an embodiment, the value may be calculated from the acceleration data measured by a sensor attached to a person's torso. For example, the acceleration data used in the calculation may be from sensor 103, which is attached above the person's sternum. In an embodiment, the one or more features may include an acceleration amplitude along a body axis (e.g., ML, AP, or SI axis), a jerk of the person's movement along a body axis, or a spectral edge frequency of the acceleration data along a body axis. The jerk of the person's movement may be calculated as a derivative of acceleration data along a sensor axis, and may measure steadiness of the person's movement along that axis. The spectral edge frequency, or SEF, of a signal may be calculated as the frequency below which 95% of the power of the signal is contained. In an embodiment, the one or more parameters may include statistical measures of the acceleration data, such as a mean, root mean squared (RMS), or CV of the acceleration amplitude or of the jerk along a body axis. The RMS of acceleration amplitude may measure sway in a person's movement.
At operation 211, a subset of the calculated features may be selected as input features of a classifier model. For example, sequential forward feature selection or stepwise feature selection may be used to select a subset of features that combine to best predict falls history. The technique may sequentially select features until there is no reduction in the unexplained variance (e.g., in Wilk's λ) of the classifier model. A F-test may be used as part of the technique. For example, the F-test may generate a value of a significance level of a feature to a classifier model. If the value is less than 0.05, the feature should not be entered into the model. Further, a feature should be removed from the classifier model if the value of the significance level is greater than 0.1. The removal and entry can be repeated until no feature meets the criteria for entry or removal.
In an embodiment, to provide performance measures reflecting robustness of a generated classifier model, leave-one-out cross-validation may be conducted to ensure that the selected subset of calculated features could be generalized to unseen data, so that the classifier model could be applicable to the population in general. For example, data from one FTSS participant may be used as validation data, and data from all other FTSS participants may be used as training data. The cross-validation may be repeated for each participant.
In an embodiment, to ensure that reliable and repeatable features are included in the model, test-retest reliability of each feature may be investigated. For example, if sensor data had been collected over multiple sessions, intra-class correlations, or ICC(2,k), may be calculated for the data collected over the multiple sessions. Features with ICC>0.4 may be considered acceptable for inclusion in the classifier model.
Table 2 illustrates example ICC values for various acceleration-based parameters.










TABLE 2







Feature
ICC (95% CI)




















Mean jerk ML stand-sit
0.93
(0.88:0.96)



Mean jerk AP stand-sit
0.92
(0.87:0.96)



CV RMS AP SSS
0.90
(0.83:0.94)



CV RMS AP sit-stand
0.89
(0.81:0.94)



Mean jerk SI stand-sit
0.88
(0.80:0.94)



Mean jerk SI sit-stand
0.87
(0.77:0.93)



Mean jerk AP SSS
0.86
(0.77:0.93)



CV SEF ML sit-stand
0.86
(0.69:0.95)



Mean jerk AP stand
0.85
(0.74:0.92)



CV SEF AP sit-stand
0.84
(0.64:0.94)



CV SEF SI sit-stand
0.81
(0.57:0.93)



Mean jerk SI SSS
0.79
(0.65:0.89)



Mean SEF ML sit-stand
0.79
(0.64:0.88)



Mean SEF SI sit-stand
0.77
(0.61:0.87)



Mean SEF AP sit-stand
0.76
(0.59:0.87)



CV SEF SI SSS
0.76
(0.59:0.87)



RMS SI total
0.73
(0.55,0.85)



CV SEF ML stand-sit
0.73
(0.33:0.92)



CV SEF ML SSS
0.73
(0.54:0.85)



CV RMS SI SSS
0.72
(0.52:0.85)



CV RMS ML stand-sit
0.72
(0.52:0.85)



RMS AP total
0.72
(0.52:0.84)



CV RMS SI sit-stand
0.71
(0.52:0.84)



Mean jerk ML stand
0.71
(0.51:0.84)



Mean SEF ML stand-sit
0.71
(0.51:0.84)



Jerk AP total
0.70
(0.49:0.84)



Mean SEF AP SSS
0.70
(0.49:0.83)



CV SEF AP SSS
0.68
(0.47:0.83)



Mean SEF SI SSS
0.68
(0.46:0.82)



Mean RMS AP stand-sit
0.67
(0.44:0.82)



Jerk SI total
0.66
(0.43:0.82)



Mean jerk ML SSS
0.66
(0.43:0.82)



Mean SEF AP stand-sit
0.66
(0.43:0.81)



CV RMS ML SSS
0.66
(0.43:0.81)



Mean SEF SI stand-sit
0.66
(0.43:0.81)



CV RMS SI stand-sit
0.66
(0.42:0.81)



CV RMS AP stand-sit
0.65
(0.40:0.81)



CV SEF AP stand-sit
0.65
(0.11:0.89)



Mean RMS SI sit-stand
0.64
(0.40:0.81)



Mean RMS AP sit-stand
0.64
(0.38:0.80)



Mean SEF ML SSS
0.64
(0.38:0.80)



Mean RMS SI stand-sit
0.63
(0.37:0.80)



Mean RMS SI SSS
0.62
(0.36:0.79)



Mean RMS AP SSS
0.60
(0.32:0.78)



Mean SSS time
0.59
(0.31:0.78)



Total time
0.58
(0.30:0.77)



CV RMS ML sit-stand
0.58
(0.29:0.77)



CV SEF SI stand-sit
0.56
(−0.10:0.87)



Mean RMS ML sit-stand
0.56
(0.25:0.76)



CV jerk SI SSS
0.56
(0.25:0.76)



SEF AP total
0.55
(0.24:0.75)



Mean RMS ML stand-sit
0.51
(0.17:0.73)



CV jerk SI stand
0.51
(0.17:0.73)



Mean RMS ML SSS
0.49
(0.14:0.72)



Mean sit-stand time
0.47
(0.10:0.71)



SEF ML total
0.44
(0.06:0.70)



SEF SI total
0.35
(−0.10:0.65)



RMS ML total
0.35
(−0.10:0.64)



Mean stand-sit time
0.33
(−0.14:0.63)



CV sit-stand time
0.30
(−0.18:0.62)



Jerk ML total
0.23
(−0.30:0.58)



CV SSS time
0.19
(−0.37:0.56)



CV stand-sit time
0.18
(−0.39:0.55)



CV jerk ML stand
0.15
(−0.44:0.53)



CV jerk ML stand-sit
0.10
(−0.52:0.51)



CV jerk ML SSS
0.03
(−0.65:0.47)



CV jerk AP stand-sit
−0.02
(−0.72:0.44)



CV jerk SI stand-sit
−0.03
(−0.74:0.44)



CV jerk AP SSS
−0.25
(−1.10:0.32)



CV jerk AP stand
−1.59
(−3.38:−0.42)










Table 3 illustrates an example subset of features selected through sequential forward feature selection as input features to a classifier model.








TABLE 3





Feature
Description







Mean jerk SI sit-stand
Average jerk of the SI acceleration during


(m/s3)
each sit-stand transition.


Mean jerk ML SSS
Average jerk of the ML acceleration during


(m/s3)
each SSS component.


CV SEF SI sit-stand (%)
Variance in spectral edge frequency of the SI



acceleration for each sit-stand transition.


CV SEF ML SSS (%)
Variance in spectral edge frequency of the ML



acceleration for each SSS component.









In the example, features related to a person's sit-stand transition may be an indicator of falls risk. The sit-stand transition may relate to smoothness of a person's movement.
The classifier model may be trained to classify falls risk by associating values of the selected features derived from a sample of people performing FTSS tests with their self-reported falls history information. For example, a linear discriminant analysis may be used to calculate linear discriminant parameter values that best describes a relationship of feature values of various people and their self-reported falls history.
At operation 215, the trained classifier model may be used to estimate falls risk of people outside the sample.
In an embodiment, performance of a classifier model may be gauged through sensitivity, specificity, and accuracy of the model. Sensitivity may be calculated as a ratio of the number of fallers correctly classified as such compared to the total number of fallers. Specificity may be calculated as a ratio of the number of non-fallers correctly classified as such compared to the total number of non-fallers. Accuracy may be calculated as a ratio of the total number of test participants correctly classified compared to the total number of participants.
In an embodiment, performance of a classifier model that is based on inertial sensor data may be compared to performance of a classifier model that is based solely on age and total FTSS time. A ROC analysis may be used for the comparison. The ROC curve analysis may allow evaluation of sensitivity and specificity for positive and negative results at various cutoff point levels of a dependent variable. As illustrated in FIG. 4, a ROC curve may be generated as a graphical representation of a relationship between sensitivity and false-positive rate (e.g., 1 minus specificity) across values of an independent or predictor variable. The area under the curve (AUC) of the ROC curve may be determined and tested against a null hypothesis of no discrimination (AUC=0.50). A ROC AUC significantly greater than 0.5 may indicate greater ability to classify a subject's falls history than would be predicted by chance.
Table 4 illustrates example values of the performance metrics described above for the classifier model based on the features illustrated in Table 3. For comparison, the table also illustrates example values of the performance metrics for a classifier model based on just ages and total FTSS time of people in the test sample.










TABLE 4







Total time &




Age
iFTSS




















Accuracy
64.1%
82.1%



Sensitivity
57.9%
66.7%



Specificity
  70%
95.2%



ROC AUC
0.793
0.89










In an embodiment, a classifier model may calculate a probability that a person will experience a fall, and may output a classification based on the calculated probability. FIG. 5 illustrates example probabilities outputted by classifier models, which use the probabilities to make a classification. In the illustration, the iFTSS-based classifier model generates a probability of close to 0 for most non-fallers and a probability of close to 1 for most fallers. In contrast, the classifier model based on total FTSS time and age generates probabilities that are more spread out.
Embodiments of the present invention are applicable for use with all types of semiconductor integrated circuit (“IC”) chips. Examples of these IC chips include but are not limited to processors, controllers, chipset components, programmable logic arrays (PLA), memory chips, network chips, and the like. In addition, in some of the drawings, signal conductor lines are represented with lines. Some may be thicker, to indicate more constituent signal paths, have a number label, to indicate a number of constituent signal paths, and/or have arrows at one or more ends, to indicate primary information flow direction. This, however, should not be construed in a limiting manner. Rather, such added detail may be used in connection with one or more exemplary embodiments to facilitate easier understanding of a circuit. Any represented signal lines, whether or not having additional information, may actually comprise one or more signals that may travel in multiple directions and may be implemented with any suitable type of signal scheme, e.g., digital or analog lines implemented with differential pairs, optical fiber lines, and/or single-ended lines.
Example sizes/models/values/ranges may have been given, although embodiments of the present invention are not limited to the same. As manufacturing techniques (e.g., photolithography) mature over time, it is expected that devices of smaller size could be manufactured. In addition, well known power/ground connections to IC chips and other components may or may not be shown within the figures, for simplicity of illustration and discussion, and so as not to obscure certain aspects of the embodiments of the invention. Further, arrangements may be shown in block diagram form in order to avoid obscuring embodiments of the invention, and also in view of the fact that specifics with respect to implementation of such block diagram arrangements are highly dependent upon the platform within which the embodiment is to be implemented, i.e., such specifics should be well within purview of one skilled in the art. Where specific details (e.g., circuits) are set forth in order to describe example embodiments of the invention, it should be apparent to one skilled in the art that embodiments of the invention can be practiced without, or with variation of, these specific details. The description is thus to be regarded as illustrative instead of limiting.
Those skilled in the art will appreciate from the foregoing description that the broad techniques of the embodiments of the present invention can be implemented in a variety of forms. Therefore, while the embodiments of this invention have been described in connection with particular examples thereof, the true scope of the embodiments of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification, and following claims.

## Claims

1. A computer-implemented method for estimating falls risk, comprising:
measuring first acceleration data from a first inertial sensor and second acceleration data from a second inertial sensor, the first and second inertial sensors comprise tri-axial accelerometers and are attached to at least one person transitioning at least one times from a standing state to a sitting state or from a sitting state to a standing state, wherein the first inertial sensor is attached to the person's lower body and the second inertial sensor is attached to the person's upper body;
receiving, at one or more processors, the first acceleration data from the first inertial sensor attached to the at least one person;
receiving, at the one or more processors, the second acceleration data from the second inertial sensor attached to the at least one person;
receiving, at the one or more processors, falls history information of the at least one person;
determining, from the first acceleration data and using the one or more processors, a first value of one or more features indicating a total time for the at least one person to complete the transitioning between the sitting state and the standing state;
determining, from the second acceleration data and using the one or more processors, a second value of the one or more features indicating steadiness of movement of the at least one person, wherein a jerk of the at least one person's movement is calculated as a derivative of second acceleration data along a sensor axis to measure the steadiness of movement of the least one person along the sensor axis;
determining, from the second acceleration data and using the one or more processors, a third value of the one or more features indicating a mean, coefficient of variation, or root mean square of the second acceleration data and a fourth value of the one or more features indicating a spectral edge frequency of the second acceleration data;
generating, at the one or more processors, a classifier model based on the determined first value, the determined second value, the determined third value, the determined fourth value, and the falls history information of the at least one person;
after the generation of the classifier model, the one or more processors receiving acceleration data from inertial sensors attached to another person;
inputting a subset of the one or more features as input features of the classifier model;
calculating, via the one or more processors using both the generated classifier model and the received acceleration data from the another person, a quantitative value for a probability of a risk of falling of the another person; and
outputting a classification of likely to fall or not being likely to fall for the another person based on the calculated probability using the one or more processors.

2. The computer-implemented method of claim 1, wherein the classifier model is a linear discriminant classifier model.

3. The computer-implemented method of claim 1, wherein the one or more processors further utilize the at least one person's age to generate the classifier model.

4. The computer-implemented method of claim 1, wherein the total time is calculated as a difference between a time corresponding to a start of a first transition and a time corresponding to an end of a fifth transition.

5. The computer-implemented method of claim 1, wherein the method further comprises calculating additional values based on the first and/or second acceleration data, the additional values indicating an acceleration amplitude along a person's body axis and a mean jerk of the person's movement along the person's body axis, and wherein the method further comprises using the additional values relating to the acceleration amplitude and the mean jerk to generate the classifier model.

6. The method of claim 1, wherein the first and second inertial sensors are attached to thigh and torso of the at least one person.

7. The method of claim 1, wherein the at least one times from a standing state to a sitting state or from a sitting state to a standing state comprises at least five times from a standing state to a sitting state or from a sitting state to a standing state.

8. A non-transitory computer-readable medium having instructions that, when executed by one or more processors, cause the one or more processors to:
receive first acceleration data from a first inertial sensor attached to at least one person transitioning at least one times from a standing state to a sitting state or from a sitting state to a standing state;
receive second acceleration data from a second inertial sensor attached to the at least one person taken during the transitioning at least one times, wherein the first and second inertial sensors comprise tri-axial accelerometers;
receive falls history information of the at least one person;
determine, from the first acceleration data, a first value of one or more features indicating a total time for the at least one person to complete the transitioning between the sitting state and the standing state;
determine, from the second acceleration data, a second value of one or more features indicating steadiness of movement of the at least one person;
determining, from the second acceleration data, a third value of one or more features indicating a mean, coefficient of variation, or root mean square of the second acceleration data and a fourth value of one or more features indicating a spectral edge frequency of the second acceleration data, wherein a jerk of the at least one person's movement is calculated as a derivative of second acceleration data along a sensor axis to measure the steadiness of movement of the least one person along the sensor axis;
generate a classifier model based on the determined first value, the determined second value, the determined third value, the determined fourth value, and the falls history information of the at least one person,
wherein the first inertial sensor is attached to the person's lower body and the second sensor is attached to the person's upper body, and,
after the generating of the classifier model, receiving acceleration data from inertial sensors attached to another person;
inputting a subset of the one or more features as input features of the classifier model;
calculating, using both the generated classifier model and the received acceleration data from the another person, a quantitative value for a probability of a risk of falling of the another person; and
outputting a classification of likely to fall or not being likely to fall for the another person based on the calculated probability.

9. The non-transitory computer-readable medium of claim 8, wherein the instructions cause the one or more processors to generate a linear discriminant classifier model.

10. The non-transitory computer-readable medium of claim 8, wherein the one or more processors further utilize the at least one person's age to generate the classifier model.

11. The non-transitory computer-readable medium of claim 8, wherein the total time is calculated as a difference between a time corresponding to a start of a first transition and a time corresponding to an end of a fifth transition.

12. The non-transitory computer-readable medium of claim 8, wherein the method further comprises calculating additional values based on the first and/or second acceleration data, the additional values indicating an acceleration amplitude along a person's body axis and a mean jerk of the person's movement along the person's body axis, and wherein the method further comprises using the additional values relating to the acceleration amplitude and the mean jerk to generate the classifier model.

13. The method of claim 8, wherein the at least one times from a standing state to a sitting state or from a sitting state to a standing state comprises at least five times from a standing state to a sitting state or from a sitting state to a standing state.